1. Field of the Invention
The present invention relates to a galvanized steel sheet with a high press formability characteristic adapted for use as an automotive non-corrosion steel sheet and a method for manufacturing the same.
2. Description of the Prior Art
Recently, there has been an increasing demand for surface-treated steel sheets with high corrosion resistance in the fields of automobiles, household appliances, etc. To cope with this demand, alloy electroplated steel sheets, such as hot dip galvanized steel sheets, galvannealed steel sheets, electrogalvanized steel sheets, steel sheets electrogalvanized with Zn-Ni, etc., have been developed and placed into use.
When using these galvanized steel sheets with high corrosion resistance as automotive steel sheets, their physical or chemical properties, including press formability, weldability, chemical convertibility, etc., which are required in an automobile production line, are expected to be on levels equal to or higher than those of cold-rolled steel sheets that: have conventionally been used for the purpose.
The inventors hereof examined the press formability of those galvanized steel sheets. Thereupon, it was revealed that the galvanized steel sheets, in contrast with the conventionally used cold-rolled steel sheets, are subjected to so great a frictional resistance against a mold during press forming operation that their press formability is relatively low.
If this frictional resistance is great, the steel sheet is prevented from being smoothly introduced into those portions which are subjected to hard sliding motion, such as a bead portion of a press mold for fixing the steel sheet at the time of press forming. In an extreme case, the steel sheet may be broken. Since the proper cushion pressure range (range of cushion pressure within which the steel sheet cannot be wrinkled or broken) of the galvanized steel sheets for the press forming is much narrower than that of the cold-rolled steel sheets, the galvanized sheets are poorer in productivity. Therefore, an improvement of the frictional characteristics of the galvanized steel sheets is urgently needed.
The frictional characteristics experienced during a press forming operation are greatly influenced by the properties of the plating surface which is directly in contact with the mold. Accordingly, the frictional characteristics are tentatively improved by coating the plating surface with some material other than a zinc plating, e.g. an organic high molecular film, so that the plating surface is kept from direct contact with the mold and is lubricated.
A method for improving the formability of a steel sheet by improving the frictional characteristics of the sheet surface is disclosed in Published Examined Japanese Patent Application No. 61-26600, for example. According to this method, a specific organic high molecular film is formed on the steel sheet surface. Further, a lubricated-surface steel sheet is proposed which is coated with a film composed mainly of metallic soap or higher fatty acid wax, for example.
However, these steel sheets having the organic film thereon are subject to the following problem.
When galvanized steel sheets are used for automotive purposes, they are usually phosphated before being coated after the press forming operation. In doing this, however, the organic film partially remains on the steel sheet without being thoroughly removed by alkali degreasing as a pretreatment for the phosphating process. Therefore, a normal crystal of a phosphate on galvanized layer cannot be produced by the phosphating process. As a result, the adhesion of the coating film is lowered, so that the corrosion resistance of the coated steel sheet is deteriorated.
Disclosed in Published Unexamined Japanese Patent Application No. 1-172578, moreover, is a method in which the frictional characteristics obtained during press forming operation are improved by forming anhydrous alkaline metal salts of oxides of one or more metalloids, selected from a group of elements including boron, phosphorus, silicon, selenium, antimony, and tellurium, on a zinc-based plating. Also in this method, however, the solubility of the alkaline metal salts of the oxides of the metalloids, which are anhydrous crystals, is much poorer than that of hydrous crystals. Therefore, the film partially remains on the steel sheet without being thoroughly removed by the alkali degreasing before the phosphating process, thus entailing the same problem as aforesaid.